Portable communicators, such as cell phones, frequently utilize antennas including a helical winding. Helical windings permit a relatively long effective antenna length with a small physical antenna length. This is convenient in cell phones and other portable communicators since small physical size is beneficial and since a certain antenna length is necessary to achieve particular broadcast and reception frequencies. Accordingly, antennas are frequently formed, in whole or part, from a helical conductor. Small size also dictates that the wire used to form the helical conductor be thin. This requires the helical conductor to be encased in a protective material, since cell phone antennas are often subjected to forces which would permanently deform delicate helical windings.
The typical helical windings are formed from a thin and delicate conductive wire. Thin wires help preserve the desired small size and low weight which is desirable in portable communicators. Thin conductive wires also facilitate the low power transmission and reception functions of portable communicators.
The coating of such thin helical conductors with protective material has proved difficult. Injection molding is an efficient and widely used coating technique, but often deforms delicate helical antenna conductors. The helical winding is placed in a mold, typically while it is mounted on a core, and thermoplastic material is injected into the mold. Significant forces are applied to the helical winding during the injection, and deform the winding by changing its pitch, i.e., the spacing between windings, and causing the pitch to be nonuniform. This changes the electrical characteristics of the antenna in a manner which may vary from one antenna to the next during manufacturing. Compensation for these variances is often achieved through additional processing, such as testing and trimming to tune the antenna to a desired frequency. Even still, a significant percentage of manufactured antennas may be unsuitable for use. Obviously, this increases both the cost and difficulty of manufacturing. In addition, performance tolerances must be generous enough to accommodate the variances experienced in those antennas which are still suitable for use.
It is known to wind the helical structure around supports prior to injection to attempt to avoid deformation. Exemplary techniques are disclosed in Bumsted, U.S. Pat. No. 5,648,788, Jul. 15, 1997, and in Valimaa et al., U.S. Pat. No. 5,341,149, Aug. 23, 1994. In the first technique, a relatively complex molding process is disclosed, where a sliding bar locks a coil onto a special handle assembly for molding. The mold includes mold pads for holding the coils in place during molding. This leaves portions of the coils exposed, requiring additional processing.
Valimaa also recognizes the potential for thin helical windings to deform during injection molding, and discloses a threaded support core, used for molding of helical coils. The core is completely molded into the coil and therefore cannot control the point of injection relative to the beginning of the winding. Neither Bumsted or Valimaa recognizes or addresses the need to control this point to avoid deformation in the first few windings.
In sum, there is a need for an improved and efficient method of manufacturing a protectively coated helically wound antenna which addresses shortcomings of prior techniques. In addition, there is a need for an improved and efficient method for manufacturing such an antenna which produces a repeatable consistent helical structure, avoids deformation throughout the winding, and avoids significant post-processing trimming and tuning.